The present invention relates to a method for preparing nonwoven, fibrous mats or sheets, particularly to a method wherein a fibrous mat or sheet is prepared from a suspension of synthetic fibers in an aqueous liquid.
Nonwoven fibrous mats or sheets prepared from synthetic fibers such as glass fibers are known to be effective reinforcement for a wide variety of products including roofing shingles, built-up roofing mats, battery walls and the like.
In the preparation of such nonwoven, fibrous mats or sheets employing conventional wet process (drift deposition) techniques, a suspension of the synthetic fibers and a suspending medium, which is typically an aqueous liquid, is deposited onto a continuous moving screen or other water-permeable support. The deposited fibrous mat is dewatered using drainage and suction; with additional amounts of water being subsequently removed using squeeze rolls, presses and/or drier's felt. Typically, to provide a mat with the desired physical properties such as tensile strength, a binder is applied to the fibers and the mat heated to cure the binder.
Using such wet process techniques, it is known that the properties such as tensile strength of the fibrous mats are affected by the properties of the fibrous suspension (e.g., the uniformity of distribution of the fibers in suspension and the settling rate of the fibers from suspension) employed in the mat's preparation. For example, the mats which generally exhibit the most desirable balance of uniformity and strength properties are prepared from fibrous suspensions wherein individual fibers are uniformly distributed (i.e., suspended) throughout the suspending medium. Unfortunately, there is a tendency for the synthetic fibers to entangle or agglomerate in the suspension thereby impairing the uniformity of the fibrous mat prepared therefrom.
Heretofore, to help achieve and maintain a desired suspension, it has been suggested that the fibrous suspensions comprise a dispersant or surface active agent to help reduce the entanglement of the fibers. For example, U.S. Pat. No. 3,772,142 teaches the use of a water-soluble polymer of an N-sulfohydrocarbyl acrylamide as a fiber dispersant. Unfortunately, the disentanglement of fibers resulting from the dispersant or surface active agent is generally not sufficient to impart the desired uniformity to the fibrous mats prepared from a suspension containing such dispersant or surface active agent.
Alternatively, U.S. Pat. No. 3,391,057 discloses that fibrous mats having improved uniformity can be prepared from an aqueous suspension of fibers and a water-soluble acrylamide polymer, which suspension may optionally contain a dispersant. While the mats prepared from such suspensions generally exhibit improved uniformity with an attendant increase in tensile strength, further tensile strength increases are often desirable to provide stronger fibrous mats or to use shorter and/or less fibers to prepare mats of equivalent strength.
U.S. Pat. No. 3,630,831 teaches the use of a binding agent for nonwovens and necessitates the addition of yet another ingredient in the suspension medium beyond those needed to disperse and/or suspend the fibers. As noted in the Manual of Nonwovens, Radko Krcma, an increase in foreign materials in the slurry decreases strength of the fiber sheet. It can be shown that although viscosity of the suspension controls the formation of the sheet, the higher amount of thickener needed to achieve that viscosity, the more interference in fiber binding. Although the majority of the thickened media is removed from the fibers during processing, a portion remains entrapped between fibers. This entrapped thickener increases water sensitivity at the fiber/fiber contact and therefore decreases wet strength.
In view of the stated deficiencies of the prior art, it remains highly desirable to provide fibrous mats or sheets having the desired uniformity and strength properties.